2802 S Novak et al. /Journal of the European Ceramic Society 28(2008)2801-2807 ible neutron activation, and to develop dense, high-purity using a ZetaProbe analyser( Colloidal Dynamics, USA), while a ZetaPals instrument(Brookhaven, USA)was employed to According to Nannetti et al., hybrid techniques offer new analyse the Zp of the fibres. Surface charge was modified possibilities for the production of high-purity gas-impermeable with hydrochloric acid, citric acid (CA, Johnson Matthey SiC/SiC composites By filling approximately one third of the GmbH, Germany), tetramethyl-ammoniumhydroxide (Tmah) inter-bundle voids with SiC powder before PIP, they achieved polyethylene-imine (PEl 10.000, Alfa Aesar, Germany ), In our investigation we have looked at the feasibility of elec- Dolapix CE64(Zschimmer Schwarz, Germany ldrich)or an cetyltrimethylammonium bromide(CTAB, Sigma rophoretic deposition(EPD)as a potential technique to fully The EPD experiments were performed with suspensions con- infiltrate SiC-fibre fabric with Sic powder. The aim of the taining 25 wt % o of solids at a constant dc voltage of 60V study was to achieve the highest possible packing density of the for 5 min using steel or copper electrodes. The electrodes owder-infiltrated preform that will be subsequently densified (20 mm x 20 mm x 0.5 mm)were positioned vertically in the either by Plp or another appropriate technique. EPD cell at a distance of 2 cm. In the experiments, the Sic-fib EPD is a commonly used process for producing coatings and woven fabric was used as the deposition electrode or placed self-standing ceramic parts; it is generally recognised as a fast, front of a steel electrode. The porosity of the deposits due to bub- simple and efficient ceramic processing technique. 0-14 Several ble formation was minimised by placing a cellulose membrane in successful attempts have been reported on the use of EPD for the front of the steel cathode or by using copper as the anode. Before production of ceramic fibre-matrix composites. It has been deposition the Sic-fabrics were rinsed with acetone and dis- shown that, depending on the electrical conductivity of the fibres, tilled water or pre-treated with a sodium dioctyl-sulfosuccinate the fibre-preform can either be used directly as the deposition (SDOSS)solution. 20 The progress of the deposition was mon- electrode or it can be positioned in front of the electrode. itored by the change in the current. The bulk deposits were In spite of the clear potential and effectiveness of the EPd evaluated in terms of the final weight and the solids content, process, the mechanisms and kinetics of the formation of the while the degree of particle packing was assessed with a scan- deposit are still not fully understood. Most papers report on ning electron microscope. The microstructures of the infiltrated results achieved by a trial-and-error approach, which frequently samples were observed in the green state using optical and elec leads to uncertain reproducibility. However, in our study we used tron microscopy the approach of a thorough analysis of the effect of the aque ous suspensions' composition and their electrokinetic behaviour 3. Results and discussion within a wide pH and zeta-potential range. This experimental approach should enable the reproducible deposition of a coat- 3. 1. Characteristics of the suspensions ing or formation of a matrix material with high green density as well as the infiltration of various fibre fabrics The use of a The properties of the aqueous SiC suspensions for elec- water-based system rather than the previously studied ethanol- trophoretic deposition were analysed in undiluted form, i.e based systems', was expected to result in some advantages containing 25 wt. solids. As shown in Fig. 1, the natural lue to the higher dielectric constant of water and the wider range pH(pH4)of the SiC suspension nearly matches the IEP, of suitable surfactants. Moreover, aqueous suspensions are also which explains why the suspension is very unst more attractive because unlike ethanol-based suspensions they with HCl resulted in a significant increase in the conductivity are not sensitive to humid environments, which should result in and only slight increase in the ZP up to a maximum value of better reproducibility. The main objective of this investigation was therefore to tailor 11 mV at pH 2.8, where the stability of the suspension was still SiC suspension's properties with positive and negative particle net-surface charges in order to be able to adapt the suspensions properties for the infiltration of a particular type of Sic-fibre HCI TMAH fabric. Our work was primarily focused on a comprehensive study of the EPD process; the investigation of the subsequ uent densification of the SiC matrix produced was not part of this R 2. Experimental The substrate material used in this investigation was Tyrano 。8 SA SiC-fibre fabric(Ube Industries, Ltd, Japan). For the infil tration we used B-Sic powder BF12(H. Starck, Germany) 1234567 with an average grain size of 0.5 um. Aqueous suspensions ontaining 25 wt %o of powder were prepared by homogenise- Fig. 1. The influence of pH change on the zeta-potential (ZP) and the conduc. tion in a multidirectional mixer for 30 min. The zeta-potential tivity of an aqueous suspension of Sic powder(solids content: 25 wt%;pH ZP) of the powders was measured in undiluted suspensions adjusted by HCl and TMAH2802 S. Novak et al. / Journal of the European Ceramic Society 28 (2008) 2801–2807 sible neutron activation, and to develop dense, high-purity SiC. According to Nannetti et al.,9 hybrid techniques offer new possibilities for the production of high-purity gas-impermeable SiC/SiC composites. By filling approximately one third of the inter-bundle voids with SiC powder before PIP, they achieved less than 10% porosity; however, this is still unacceptably high. In our investigation we have looked at the feasibility of elec￾trophoretic deposition (EPD) as a potential technique to fully infiltrate SiC-fibre fabric with SiC powder. The aim of the study was to achieve the highest possible packing density of the powder-infiltrated preform that will be subsequently densified either by PIP or another appropriate technique. EPD is a commonly used process for producing coatings and self-standing ceramic parts; it is generally recognised as a fast, simple and efficient ceramic processing technique.10–14 Several successful attempts have been reported on the use of EPD for the production of ceramic fibre-matrix composites.14–16 It has been shown that, depending on the electrical conductivity of the fibres, the fibre-preform can either be used directly as the deposition electrode or it can be positioned in front of the electrode. In spite of the clear potential and effectiveness of the EPD process, the mechanisms and kinetics of the formation of the deposit are still not fully understood. Most papers report on results achieved by a trial-and-error approach, which frequently leads to uncertain reproducibility. However, in our study we used the approach of a thorough analysis of the effect of the aque￾ous suspensions’ composition and their electrokinetic behaviour within a wide pH and zeta-potential range. This experimental approach should enable the reproducible deposition of a coat￾ing or formation of a matrix material with high green density as well as the infiltration of various fibre fabrics. The use of a water-based system rather than the previously studied ethanol￾based systems17,18 was expected to result in some advantages due to the higher dielectric constant of water and the wider range of suitable surfactants. Moreover, aqueous suspensions are also more attractive because unlike ethanol-based suspensions they are not sensitive to humid environments, which should result in better reproducibility.19 The main objective of this investigation was therefore to tailor SiC suspension’s properties with positive and negative particle net-surface charges in order to be able to adapt the suspension’s properties for the infiltration of a particular type of SiC-fibre fabric. Our work was primarily focused on a comprehensive study of the EPD process; the investigation of the subsequent densification of the SiC matrix produced was not part of this work. 2. Experimental The substrate material used in this investigation was Tyrano SA SiC-fibre fabric (Ube Industries, Ltd., Japan). For the infil￾tration we used -SiC powder BF12 (H. Starck, Germany) with an average grain size of 0.5 m. Aqueous suspensions containing 25 wt.% of powder were prepared by homogenisa￾tion in a multidirectional mixer for 30 min. The zeta-potential (ZP) of the powders was measured in undiluted suspensions using a ZetaProbe analyser (Colloidal Dynamics, USA), while a ZetaPals instrument (Brookhaven, USA) was employed to analyse the ZP of the fibres. Surface charge was modified with hydrochloric acid, citric acid (CA, Johnson Matthey GmbH, Germany), tetramethyl-ammoniumhydroxide (TMAH), polyethylene-imine (PEI 10.000, Alfa Aesar, Germany), cetyltrimethylammonium bromide (CTAB, Sigma–Aldrich) or Dolapix CE64 (Zschimmer & Schwarz, Germany). The EPD experiments were performed with suspensions con￾taining 25 wt.% of solids at a constant dc voltage of 60 V for 5 min using steel or copper electrodes. The electrodes (20 mm × 20 mm × 0.5 mm) were positioned vertically in the EPD cell at a distance of 2 cm. In the experiments, the SiC-fibre woven fabric was used as the deposition electrode or placed in front of a steel electrode. The porosity of the deposits due to bub￾ble formation was minimised by placing a cellulose membrane in front of the steel cathode or by using copper as the anode. Before deposition the SiC-fabrics were rinsed with acetone and dis￾tilled water or pre-treated with a sodium dioctyl-sulfosuccinate (SDOSS) solution.20 The progress of the deposition was mon￾itored by the change in the current. The bulk deposits were evaluated in terms of the final weight and the solids content, while the degree of particle packing was assessed with a scan￾ning electron microscope. The microstructures of the infiltrated samples were observed in the green state using optical and elec￾tron microscopy. 3. Results and discussion 3.1. Characteristics of the suspensions The properties of the aqueous SiC suspensions for elec￾trophoretic deposition were analysed in undiluted form, i.e., containing 25 wt.% solids. As shown in Fig. 1, the natural pH (pH ∼ 4) of the SiC suspension nearly matches the IEP, which explains why the suspension is very unstable. Titration with HCl resulted in a significant increase in the conductivity and only slight increase in the ZP up to a maximum value of 11 mV at pH 2.8, where the stability of the suspension was still Fig. 1. The influence of pH change on the zeta-potential (ZP) and the conduc￾tivity of an aqueous suspension of SiC powder (solids content: 25 wt.%; pH adjusted by HCl and TMAH).